An electroluminescent display, such as an organic light-emitting diode (OLED) display, has the advantages of light weight, wide viewing angle, bright color, fast picture response, low energy consumption and flexible processing. It is considered to be one of the most attractive emerging technologies. In principle, positive carriers and negative carriers are injected into an organic semiconductor layer (i.e., light-emitting layer) and combined together so that the organic light-emitting diode (OLED) display emits light